1. Field of the Invention
The present invention relates to a mobile communication system and, in particular, to a method and apparatus for performing and controlling measurement of a terminal in a mobile communication system capable of carrier aggregation.
2. Description of the Related Art
Mobile communication systems have developed to provide the subscribers with voice communication services on the move. With the advance of technologies, the mobile communications have been evolved to support high speed data communication services as well as the standard voice communication services. Recently, as one of the next generation mobile communication system, Long Term Evolution (LTE) is on the standardization by the 3rd Generation Partnership Project (3GPP). LTE is a technology designed to provide high speed packet-based communication of up to 100 Mbps and standardized almost completely now with the aim at commercial deployment around 2010 timeframe. Meanwhile, unlike voice service, the data service is provided on the resource determined according to the data amount to be transmitted and channel condition. Accordingly, the radio communication system, especially cellular communication, is provided with a scheduler which manages transmission resource allocation in consideration of the required resource amount, channel condition, data amount, etc. This is also the fact in the LTE system, as the next generation mobile communication system, and the scheduler located at the base station manages the transmission resource allocation.
As the LTE standard is on the verge of ratification, discussion is focused on LTE-advanced (LTE-A) with the adoption of various novel techniques to LTE. One of the key technologies adopted to LTE-A is Carrier Aggregation (CA). CA is a technique to use multiple uplink and multiple downlink carriers in data transmission unlike the conventional single carrier transmission performed with one uplink carrier and one downlink carrier. By allocating resource to a terminal on multiple carriers, it is possible to increase the transmission speed/data rate for the terminal.
FIG. 1 is a diagram illustrating the architecture of an LTE or LTE-A mobile communication system.
Referring to FIG. 1, the radio access network of an LTE/LTE-A system includes evolved Node Bs (eNBs) 105, 110, 115, and 120, a Mobility Management Entity (MME) 125, and a Serving-Gateway (S-GW) 130. The User Equipment (hereinafter, referred to as UE) 135 connects to an external network via eNBs 105, 110, 115, and 120 and the S-GW 130.
The eNBs 105, 110, 115, and 120 correspond to legacy node Bs of Universal Mobile Communications System (UMTS). The eNBs 105, 110, 115, and 120 allow the UE to establish a radio link and are responsible for complicated functions as compared to the legacy node B. In the LTE system, all the user traffic including real time services such as Voice over Internet Protocol (VoIP) are provided through a shared channel and thus there is a need of a device which is located in the eNB to schedule data based on the state information such as UE buffer conditions, power headroom state, and channel state. Typically, one eNB controls a plurality of cells. In order to secure the data rate of up to 100 Mbps, the LTE system adopts Orthogonal Frequency Division Multiplexing (OFDM) as a radio access technology on up to 20 MHz bandwidth. Also, the LTE system adopts Adaptive Modulation and Coding (AMC) to determine the modulation scheme and channel coding rate in adaptation to the channel condition of the UE. The S-GW 130 is an entity to provide data bearers so as to establish and release data bearers under the control of the MME 125. MME 125 is responsible for various control functions and connected to a plurality of eNBs 105, 110, 115, and 120.
FIG. 2 is a diagram illustrating a protocol stack of the 3GPP LTE/LTE-A system.
Referring to FIG. 2, an eNB operates with multiple carriers on different frequency bands for transmission and reception. Assuming downlink carrier_1 201 having its center frequency F1 and bandwidth BW1, downlink carrier_2 203 having its center frequency F2 and bandwidth BW2, and downlink carrier_3 205 having its center frequency F3 and bandwidth BW3; the legacy UE can receive signals on one of these downlink carriers while the UE capable of CA can receive signals on multiple carriers.
That is, the UE capable of CA can receive the signals on the downlink carrier_1, downlink carrier_2, and downlink carrier_3 simultaneously. Likewise, the legacy UE can transmit signals on one of the uplink carriers, while the UE capable of CA can transmit signals the uplink carrier_1 211, uplink carrier_2 213, and uplink carrier_3 215 simultaneously. If necessary, the eNB allocates to the UE capable of CA the more resource on multiple carriers to increase the downlink/uplink transmission speed/data rate. By taking notice that a cell is configured with one downlink carrier and one uplink carrier in a conventional concept, the carrier aggregation can be understood as if the UE communicates data via multiple cells. With the use of carrier aggregation, the maximum data rate increases in proportion to the number of aggregated carriers.
FIG. 3 is a flowchart illustrating the measurement operation of the UE for Smeasure in the conventional 3GPP LTE system.
Referring to FIG. 3, the Smeasure value is transmitted by the eNB in the message carrying the serving or neighbor cell measurement configuration information. If the Smeasure and intra-Frequency (intra-F) measurement, inter-Frequency (inter-F), and inter-Radio Access Technology (inter-RAT) neighbor cells' measurement configuration information are received at step 301, the UE performs measurement to the serving cell at step 311 and then, if the serving cell's channel measurement result value is greater than Smeasure value at step 321, suspends performing intra-F, inter-F, and inter-RAT neighbor cell measurements at step 331. This is because when the channel condition of the serving cell is good enough there is no need of handover, and thus it is possible to avoid unnecessary measurement to the neighbor cells, resulting in power saving. Otherwise, if the serving cell's channel measurement result value is not greater than Smeasure value at step 321, the UE performs, as configured at step 301, the intra-F, inter-F, and inter-RAT neighbor cell measurements at step 341.